Solar energy collectors

ABSTRACT

This invention utilizes a solar radiation absorbing fluid (i.e.: a black fluid) for direct solar energy to thermal energy transfer. The unique structure of the several disclosed embodiments of this invention provides for a selected distribution and a selected depth of the absorbing fluid in the solar energy exposed area. 
     The disclosed apparatus converts solar radiation energy into thermal energy with higher conversion efficiency, lower material and manufacturing costs and considerable weight reduction compared to conventional solar collectors.

The invention described herein may be manufactured, used, by or for theGovernment for Governmental purposes without the payment to me of anyroyalties thereon.

BACKGROUND OF THE INVENTION

This invention relates to solar energy collectors for converting solarenergy to thermal energy, and more particularly, to solar collectors inwhich a black fluid is used as the absorber and as the heat carrier.

Solar energy collectors for converting solar energy to thermal energyare well known in the art and many various different types of suchcollectors are disclosed in the prior art. Generally, such absorbersutilize thin black surfaces coated on a heat conducting metallicsubstrate such as copper, aluminum or steel. The thin surface coatingwhich is absorbing in the wavelength of the solar spectrum converts thesolar energy to thermal energy which is transferred into and through themetallic substrate into a flowing fluid, usually water or a mixture ofwater and an anti-freeze. This flowing fluid provides the necessarytemperature differential gradient and is used to transport the thermalenergy to the point of use.

Recently, solar collectors have been introduced which provide animprovement over the above described prior art devices by replacing thethin absorbing coatings and metallic substrate with a heat absorbingblack fluid which functions as both the absorber and the medium fortransporting the thermal energy to the point of use. It will beappreciated that the substitution of a black fluid in solar collectorsof conventional design may be advantageous and that solar collectorsespecially designed for black fluid use may afford optimum advantage.

SUMMARY OF THE INVENTION

This invention provides solar energy flat plate collectors of uniqueconfiguration wherein the solar energy absorber and thermal energycarrier is a radiation absorbing fluid. The collector hardware consistsof a minimal number of components which are readily available or easilyfabricated and of reasonable cost. The radiation absorbing fluidconsists of suitable carrier liquid containing suspended, dispersed ordissolved radiation absorbing materials which are active in the totalrange of the solar spectrum and therefore appear optically black. Thecarrier liquid can be water or water mixtures with organic liquids suchas ethylene glycol or glyzerin, which are added for the purpose oflowering the freezing point or raising the boiling point, or it can beany suitable oil, like silicone oil. The radiation absorbing materialsadded to the carrier liquid may consist of carbon or metal blacks orcompounds and dyes usually containing metallo-complex groups which actas radiation absorption centers.

A typical example of a fluid that has been used and operatessatisfactorily at low temperatures is China ink. China ink normallyconsists of Carbon plus a binder suspended in water. For hightemperature operation water solutions and dispersions of Acidol andVialon dyes have been successfully used (recorded in the InternationalColor Index as No. 187, No. 194 and No. 63). One of the most importantfeatures of the collector of this invention is that the absorptioncenter density or concentration in regard to the collector geometry maybe chosen in such a way as to allow essentially total absorption by thefluid. Some absorption by the intermediate glass covering is inherent,of course, but such intermediate surface absorption should be minimizedas it is efficiency derating.

It will be appreciated that relatively complex thermal considerationsare involved in each disclosed embodiment of this invention. In generaldissertation, however, radiation incident on any medium will be partlyreflected and the rest whose radiation intensity shall be φ_(o) willpenetrate the medium. If φ_(o) does not change with penetration depth,the medium is called transparent. If the radiation intensity decreases,the medium is called absorbing. In absorption, radiation energy isliterally destroyed and converted into thermal energy. In the presentinvention, of course, where the fluid is the absorbing medium, depth ofthe fluid is a significant consideration.

The absorption coefficient of the fluid medium in this invention isespecially significant for optimum design of the solar collector. Itwill be seen that radiation intensity φ decreases with increasing depthl from the surface of the medium. If we assume the radiation intensityto be φ at the depth l, then it will change to φ-dφ/dl dl at l+dl. Thismeans the decrease of intensity per unit length is -dφ/dl, which in turnis proportional to the instant intensity φ, or -dφ/dl=Kφ(l)Λdφ/φ=-Kdl(for K>0). Integration results in: l_(n) φ=l_(n) φ_(o) -Kl, or φ=φ_(o)e^(-Kl). K is the absorption coefficient and, naturally, a function ofthe wavelength λ of the radiation and of the specific characteristics ofthe absorbing medium, but not of l. Since absorbing along l depends onlyon the number of absorption centers of particles, the total number ofabsorption centers obviously is proportional to the product Cl, if Crepresents the concentration along l. With K=K'C, it follows thatφ=φ_(o) e^(-K'Cl). Accordingly, in this invention, a selected absorptioneffect can be obtained both in the case where the medium has a smallconcentration of absorption centers and great depth and in the case of ahigh concentration and small penetration depth.

BRIEF DESCRIPTION OF THE DRAWING

A complete understanding of the exact nature and structural detail ofthe invention can be obtained from the following detailed descriptionwhen read in conjunction with the annexed drawing in which:

FIG. 1 is a top view of a first embodiment of the invention;

FIG. 2 is a transverse cross section, of the first embodiment taken online 2--2 of FIG. 1;

FIG. 3 is a longitudinal, in cross section, of the first embodimenttaken along line 3--3 of FIG. 2;

FIG. 4 is a top view of a second embodiment of the invention;

FIG. 5 is a transverse cross section, of the second embodiment takenalong line 5--5 of FIG. 4;

FIG. 6 is a longitudinal cross section, of the second embodiment takenalong line 6--6 of FIG. 5;

FIG. 7 is a longitudinal cross section, of a third embodiment of theinvention;

FIG. 8 is a schematic showing of a typical closed cycle solar energysystem in accordance with the present invention; and

FIG. 9 is a graphical showing of the relationship of collector geometryand absorption center concentration in the fluid for total absorption.

DESCRIPTION OF THE INVENTION

FIGS. 1, 2 and 3 show a first embodiment of the invention. Thisembodiment is designed for flat or tilted deployment.

As shown in FIGS. 1, 2 and 3, this embodiment comprises a trough orhousing 1 made of rigid insulating material such as polyurethane moldedin one single piece and coated with a thin ultraviolet radiation andwater resistant coating such as white epoxy primer and paint. The whitecolor of the paint provides for maximum radiation reflection of allcontinually exposed surfaces and prevents overheating of the collectorwhen the fluid is drained for repair purposes, for example. Further,when the collector of this embodiment is used as a roof mountedcollector, housing or trough 1 enables assembly of several collectors toprovide a waterproof roof structure. For example, the collectors may beprovided either with leak tightened plastic strips interconnectingsurrounding collector housings or outer edges of each collector housingmay be molded to allow staggered deployment (or overcutting) similar tofired clay roof tiles. As various means of interconnection, well knownin the prior art, may be utilized to assemble panels into a waterproofstructure, no specific interconnection is shown in the drawings.

The open top trough 1 is enclosed by an outer transparent cover 2 madeof glass or in hail storm or vandal prone areas of an impact resistant,rigid or flexible polymer sheet, such as LEXAN. Outer transparent cover2 rests on and is secured to a ridge or plateau 3 formed in trough 1.

A plurality of parallel channels 4 extending along the length of trough1 are secured inside trough 1. A black fluid 5 which serves as theabsorber and heat carrier of the collector flows through channels 4.Black fluid 5 is fluid made up of suspension, dispersion or solution ofradiation absorbing materials such as Carbon, Vailon and Acidol in thecarrier fluid which can be water or any other suitable liquid, forexample, glycerine in applications where a higher boiling point isdesirable. Both ends of each channel 4 have a plurality of openings 6which form a manifold inlet or outlet depending upon the flow directionof black liquid 5. Opening 6 in one end of each channel 4 communicatewith the manifold channel 7 and opening 6 of the other end of eachchannel 4 communicate with identical manifold channel 8. The pipe 9communicates with manifold channel 8 and the identical pipe 10communicates with manifold channel 7. A feature of this invention isthat it enables the use of low cost piping material, for example,readily available rigid PVC pipe of the type used in DWV plumbingapplications. It will be appreciated that the minimal pressureconstraints, characteristic of the invention enable the use of DWVpiping, if desired. To insure proper flow for optimum efficiency, pipes9 and 10 must be at diagonally opposite corners. Pipes 9 and 10 areshown as passing through the under surface of trough 1. It will beappreciated that the pipes 9 and 10 could, instead, pass through theside walls.

An inner optically transparent inner glass cover 11 made of Float-glassor PYREX rests on and is sealed to the ridge or pleateau 12 formed intrough 1. Inner glass cover 11 serves as an infra-red filter and as thetop retaining cover for black fluid 5. By sealing inner glass cover 11to plateau 12 provided in trough 1, protection against leakage of blackfluid 5 from a closed cycle system such as the system shown in FIG. 8 isinsured. Inner glass cover 11 also rests on and is sealed to a pair ofwidened ridges 13. Wide ridges 13 protect inner glass 11 from breakagedue to sudden pressure surges of black fluid 5 during pumpinginitiation. While only two ridges 13 are shown any number of suchridges, conveniently uniformly spaced, can be provided. The spacebetween inner glass cover 11 and outer cover 2 is a dead air space andthe main functions of outer cover 2 are to provide this dead air spaceto minimize heat losses by convection and to lower losses of infra-redradiation energy.

The collector of FIGS. 1, 2 and 3 is typically used in a system such asthe system shown in FIG. 8. In FIG. 8, the collector 14 represents oneor more of the collectors of this invention. Thus, the collector ofFIGS. 1, 2 and 3 may be collector 14 in FIG. 8. Assuming this is thecase, conduit 15 would be coupled to either pipe 9 or pipe 10 andconduit 19 would be coupled to the other pipes 9 or 10. A pump 18 pumpsblack fluid 5 in the direction indicated by the arrow. When thecollector is deployed in a tilted fashion, black fluid 5 is pumped orthermosyphoned upward. If collector 14 were horizontal, black fluid 5could be pumped in either direction. Thus, black fluid 5 passes upwardthrough collector 14 in channels 4 in this case since it is assumed thatcollector 14 is the collector of FIGS. 1, 2 and 3 and out of collector14 via conduit 15. While black fluid 5 passes through collector 14, itabsorbs the solar energy impinging on collector 14 and converts thissolar radiation energy to thermal energy. The thermal energy is carriedby black fluid 5 as sensible heat that is based on the black fluid'sheat capacity via conduit 15 to heat exchanger 16 which extracts theheat from black fluid 5. Black fluid 5 with the heat extracted iscarried to pump 18 via conduit 17. Pump 18 pumps black fluid 5 in thedirection of the arrow back through collector 14 via conduit 19 for thenext cycle. Thus, black fluid 5 acts not only as the absorber in thesystem of FIG. 8 but also as the vehicle for transporting the thermalenergy to heat exchanger 16 (the point of use of the thermal energy).

Referring to FIGS. 4, 5, and 6, these Figures show a second embodimentof the invention. Again, a one-piece molded housing or trough 20 made ofa rigid insulating material that has been weather and ultra-violetradiation proofed is provided. Housed inside trough or housing 20 is aspecially designed, flat, transparent, and ultra-violet radiationresistant flexible polymer (polycarbonate, POLYVINYL, TEDLAR or thelike) bag 22. Bag 22 consists of an inner bag 24 welded together atintervals to form the channels 25 and a cover 23 which covers the top ofinner bag 24 and is secured on all four sides to inner bag 24 in such amanner that an air tight seal is formed. Cover 23 is made from aflexible transparent polymer and inner bag 24 also may be made from aflexible transparent polymer. Each of the channels 25 has a manifold ateach end. In FIG. 5, the manifolds are indicated as the holes oropenings 26. However, in this embodiment, the manifold design is notcritical and any suitable manifold arrangement may be used. For example,separate holes or sleeves made of a material that is readily faced tothe material of inner bag 24 could be secured to the ends of each of thechannels 25. The holes and sleeves would be sealed at the end oppositethe end secured bag 24 and would have a plurality of holes along theirlengths. The manifold arrangements provided at each end of each of thechannels 25 are either inlet manifolds at one end of channels 25 andoutlet manifolds at the other end of channels 25 or vice-versa dependingupon the direction of flow of the black fluid 27 which flows through thechannels 25. Black fluid 24 is identical to black fluid 5 used in thefirst embodiment shown in FIGS. 1, 2 and 3 and also serves as both theabsorber and vehicle for transporting the thermal energy to the point ofuse.

Trough or housing 20 is closed by means of a transparent top sheet 21 offlexible or rigid polymer that is secured along the entire top edge oftrough or housing 20 but for primitive applications, no housing or topsheet are required. An air cushion is formed on the top of inner bag 24by means of cover 23. An air valve 28 is provided in cover 23 to providepressure control of the air pressure on inner bag 24 and thus channels25.

A pipe or channel 29 communicates with the manifolds 26 at one end ofchannels 25 and a second pipe or channel 30 communicates with themanifolds at the other end of channels 25. In FIG. 6 the manifold pipe29 is illustrated to be external to inner bag 24. Pipe 29 is carried outthrough one side of trough or housing 20 adjacent the corner 31 oftrough or housing 20 and pipe 30 is carried out through the other sideof trough or housing 20 adjacent the corner 32 of trough or housing 20,corners 31 and 32 being diagonally opposite corners.

FIG. 7 is a longitudinal cross section of a third embodiment of theinvention. This third embodiment is merely a variation of the secondembodiment of the invention; therefore, like numerals have been used toindicate like parts. In the embodiment, manifold pipe 29 is internal toinner bag 24. This merely requires a different sealing pattern.

The collectors of FIGS. 4, 5, 6 and 7 each alone or in combination mighttypically be used as collector 14 in a system such as the system of FIG.8. The system of FIG. 8 operates in the same manner when the collectorof FIGS. 4, 5, 6 and 7 is used for collector 14 as it does when thecollector of FIGS. 1, 2 and 3 is used. Note, the collectors of FIGS. 4,5, 6 and 7 will typically be used in a flat or horizontal position butmay be used in slightly tilted positions at relatively small angles, ifdesired.

While the invention has been described with reference to severalparticular embodiments, it will be obvious to those skilled in the artthat various changes and modifications can be made to each of theembodiments shown and described without departing from the spirit andscope of the invention as set forth in the claims.

What is claimed is:
 1. A solar collector for radiant energy exposure ofa selected fluid medium having a high radiation absorption coefficientin a low pressure, moving fluid medium, energy transfer systemcomprising:an elongated input manifold means; an elongated outputmanifold means; a plurality of parallel fluid medium channels, saidchannels disposed in parallel and in planar alignment and disposed tointerconnect said input manifold means and said output manifold means,said channels having a substantially uniform length and a substantiallyidentical internal cross section, each of said channels having asubstantially uniform internal cross section along its length with adetermined planar depth internal dimension, said channels transverselyoriented to said input manifold means and to said output manifold meanssuch that said elongated input and output manifold means are disposedalong the respective lengths thereof in the plane of said plurality ofparallel channels; said input manifold means and said output manifoldmeans including fluid medium input means and fluid medium output means,respectively, each of said input means and said output means disposedand adapted to produce an equibalanced pressure differential across saidplurality of parallel channels; said planar depth internal dimension ofsaid channels being inversely proportional, for any given radiant energyintensity, to the radiation absorption coefficient of said fluid medium.2. A solar collector for radiant energy exposure of a selected fluidmedium, having a high radiation absorption coefficient, in a lowpressure, moving fluid, energy transfer system comprising;an opaque backmember of substantially planar and substantially rectangularconfiguration and a substantially transparent front member ofsubstantially planar and substantially rectangular configuration inspaced relation with two parallel elongated side sections defining ahollow chamber with first and second open ends and having asubstantially uniform front to back depth, said hollow chamber includinga plurality of elongated divider means each extending from said firstopen end to said second open end and defining a plurality of channelswithin said hollow chamber; first and second manifold means coupled tosaid first and second open ends, respectively, of said hollow chamberand defining a substantially rectangular fluid container adapted tocontain said fluid medium, said manifold means including fluid inputmeans associated with said first manifold means and disposed in thevicinity of a first corner of said substantially rectangular fluidcontainer and fluid output means associated with said second manifoldmeans and disposed in the vicinity of a second corner diagonallyopposite said first corner, said fluid input means and said fluid outputmeans connected to said first and second manifold means, respectively,such that fluid entering and exiting respecting manifolds thereby movesin a direction orthogonal to the direction of movement of fluid in saidplurality of channels defined by said elongated dividers; said fluidinput means and said fluid output means connected to said energytransfer system such that said substantially rectangular fluid containeris an integral element of said system with said fluid medium of saidsystem moving therethrough, said substantially rectangular fluidcontainer being the radiant energy exposure element of said system anddisposed with said transparent first member oriented to receive radiantenergy from the solar energy source, said fluid container adapted forfull fluid operation and said front to back depth thereof beinginversely proportional, for any given radiant energy intensity, to theradiation absorption coefficient of said fluid medium.
 3. A solar energycollector as defined in claim 2 wherein said opaque back member is asingle moulded piece of fluid medium impervious material which includesall, but the top side, of each of said first and second manifolds andalso includes said elongated side sections and said plurality ofelongated divider means, said single moulded piece adapted to enable afluid medium impervious seal of said front member with said back member.4. A solar energy collector as defined in claim 3 wherein said singlemoulded piece is moulded of insulating rigid foam material.
 5. A solarenergy collector as defined in claim 4 wherein said single moulded pieceis coated with a weather and UV radiation resistant material.
 6. A solarenergy collector as defined in claim 5 wherein said weather and UVradiation resistant material is an epoxy paint.